Many work machines, particularly earth working machines, use a continuously variable transmission to drive traction wheels or tracks of the work machine. Typically, a hydrostatic drive, consisting of a hydraulic pump and a hydraulic motor, provides a continuously variable speed output to the wheels or tracks of the work machine. In particular, the speed output can be continuously varied by controlling the displacements of either the hydraulic pump or the hydraulic motor which comprise the hydrostatic drive system.
In order to operate over a wide range of operating conditions, the output of the continuously variable hydrostatic transmission is transmitted through a mechanical transmission. The mechanical transmission has a number of transmission ranges corresponding to different operating speeds of the work machine. The combination of the continuously variable transmission and the mechanical transmission allows a continuously variable transmission to operate over a wider range of speeds than is possible using the continuously variable transmission alone.
One drawback to using a continuously variable transmission with a number of transmission ranges is that power may not be smoothly transmitted through the transmission due to a range shift from a first gear range to a second gear range. Typically, during the range shift, there is a torque reversal in the continuously variable transmission. When the continuously variable transmission is a hydrostatic transmission, the actuators which control the displacement of a variable displacement hydraulic components have the slowest response time. Thus, during a range shift, the displacement remains relatively constant during the torque reversal. The torque reversal and the relatively constant displacement can cause the work machine to be perceived as non-continuous during the range shift and thus, the range shift may feel objectionable if the shift point is not adjusted for load.
However, it is possible to advantageously choose shift points such that reversal of torque is taken into consideration and the displacement of a controlling variable displacement hydraulic device, either the pump or the motor, remains relatively constant during the shift. However, to choose such advantageous shift points, it is necessary to estimate the volumetric efficiency of the components of the hydrostatic transmission in order to determine which shift points result in the same displacement after the shift as before the shift with no discrete change in output speed.
What is needed therefore is a method and apparatus for adjusting the shift point as a function of load when shifting ranges in a continuously variable transmission which overcomes the above-mentioned drawbacks.